1. Field of the Invention
The present invention relates to the electrode construction of a semiconductor disc element, and more particularly, to the electrode construction of a disc semiconductor element having a semiconductor chip held between electrode plates which are greater in diameter than the semiconductor chip and are joined to the respective sides of the semiconductor chip with solder.
2. Discussion of the Related Art
A conventional electrode construction of the above semiconductor disc element will be described with reference to FIGS. 20 to 23 using a Silicon Surge Absorber as an example. As shown in FIGS. 20 to 23, the semiconductor element comprises a square semiconductor chip 1, disc electrode plates 2 used to hold the semiconductor chip 1 therebetween and joined to the respective sides thereof with solder, solder layers 3, and passivation (silicon oxide layer) 5 approximately several micrometers thick and formed on respective sides of the semiconductor chip 1 along a junction 4 where p-type and n-type regions contact.
Each flat electrode plate 2 is soldered to the semiconductor chip 1 with the area inside the passivation 5 defining a solder joint 6. When the semiconductor chip 1 is soldered to the electrode plates 2, a jig is used to keep the temporary assembly intact by inserting a solder sheet between the semiconductor chip 1 and each of the electrode plates 2 and then passing the lamination through a heating oven to effect soldering.
The conventional electrode construction of a semiconductor disc element employing the above-described flat electrode plates joined onto both sides of a semiconductor chip has the following shortcomings.
(1) Because the passivation 5 of the semiconductor chip 1 hinders the electrode plate 2, the electrode plate 2 is kept slightly afloat from the solder joint of the semiconductor chip. If the amount of solder is insufficient, the solder joint therebetween, that is, the ohmic contact, will become incomplete. Consequently, the semiconductor element does not perform its characteristic functions during its operation. Moreover, if external pressure is applied via the electrode plate 2 to the semiconductor chip 1, the passivation may be cracked and damaged because the welding force is directly applied via the electrode plate 2 to the passivation 5. PA1 (2) As molten solder tends to spread over the entire area of the electrode plates 2 due to wetness of the electrode plates during soldering, the surface tension of the solder acting thereon will allow the semiconductor chip 1 to shift randomly from the central position of the electrode plates 2. In some cases, the semiconductor chip 1 may be soldered such that the edge portion thereof is forced outside the outer peripheral edge of the electrode plates 2 as shown by a dotted line in FIGS. 22 and 23. Moreover, the semiconductor element soldered in such a manner may be damaged if the chip portion protruding beyond the electrode plate is bumped while the semiconductor chip 1 is being handled. Such semiconductor chips would be inferior and undesirable. PA1 (3) Because the space between each of the electrode plates 2, disposed above and below the semiconductor chip 1, and the semiconductor chip are secured to about the extent of an insulating distance equivalent to the thickness of the semiconductor chip, an electric discharge may occur between the electrode plates when the surge voltage is applied, irrespective of the characteristics of the semiconductor chip 1. As a result, the semiconductor chip 1 (surge absorber) may not be able to perform the surge absorbing function adequately. PA1 (4) Conventionally, liquefied silicone rubber has been applied to the peripheral sides of the semiconductor chip 1 and hardened to protect the semiconductor chip 1 from external forces. This is, however, extremely difficult and impractical. Unless meticulous care is taken when applying the liquefied silicone rubber, the silicone rubber may be applied inadvertently to an undesired portion such as the surface of the electrode plate, in which case the thickness of the layer of the semiconductor chip 1 may become thin.